RV Electrical Safety: Part VI – Voltage Drop

The No~Shock~Zone: Part VI – Voltage Drop

If you’ve read the survey we did July 2010 in www.RVtravel.com, you know that 21% of RV owners who responded have been shocked by their vehicle. Review the 21% report at http://new.noshockzone.org/15/. What follows is #6 in a 12-part series about basic electricity for RV users and how to protect yourself and your family from shocks and possible electrocution.

This series of articles is provided as a helpful educational assist in your RV travels, and is not intended to have you circumvent an electrician. The author and the HOW-TO Sound Workshops will not be held liable or responsible for any injury resulting from reader error or misuse of the information contained in these articles. If you feel you have a dangerous electrical condition in your RV or at a campground, make sure to contact a qualified, licensed electrician.

Rumors and Innuendo

We’ve all heard about how hooking up an RV on too long or too skinny of an extension cord can force its appliances to run on 100 volts instead of the regular 120 volts, thereby burning out the motors or other components. While this may happen only rarely in your home, that’s because the electric company works very hard to keep the voltage levels constant no matter how much current you’re drawing. However, that may not be the case when you’re using your own extension cord running from a campsite pedestal.

But before we get into the reality of what happens to electrical gear that’s running on 100 volts rather then a full 120 volts, let’s figure out why this voltage drop thing happens in the first place.

From the Beginning

We’re going to put together the concepts you’ve learned about voltage in Article I and amperage in Article V in this NSZ-RV series. If you’ve not read them already, then please start at the beginning and spend an hour reading parts I through V. [See http://new.noshockzone.org/category/rv-safety/ ] Consider this time an investment in your family’s safety. Even if you know how to run a digital voltmeter, please re-read Part II on meters since that’s important to your understanding of how current draw causes voltage drop.

What’s This Voltage Drop Thing?

If you look at the first illustration you’ll see a pump on your left that can supply 120 PSI (Pounds per Square Inch) of pressure, and two pipes heading to the right side. I’ve capped one pipe with a white stopper and the other with a black stopper so that no water can leak out. Because there’s no water flow, the pressure within each pipe will be equal to the pressure of the pump. The bottom pipe, which is hooked to the 120 PSI output of the pump, will have 120 PSI all along its length, while the top pipe, which drains back into the pump, will have 0 PSI along its entire length. And you can imagine that it really doesn’t matter if that pipe is large or small in diameter. The pressure within each pipe will be equal throughout its length. I’ve added a differential pressure gauge to the far right of the illustration that shows there’s now 120 PSI difference between the two pipes, just like a voltmeter reads the voltage difference between its two probes. Note that no real work is being done; it’s just an equalized pressure system. This is exactly what happens to an electrical outlet in your home or RV. There’s electrical pressure (Voltage) but no current flow (Amperage) until you plug something into it.

Big Pipes Equal Small Pressure Loss

Now let’s make our pump do some work. I’ve added a small turbine to the right side of the drawing connecting the top pipe to the bottom pipe. The pressure of the water will cause a current to flow through that small turbine to power your blender making a frozen drink of your choice. But this isn’t a perfect world, and because there are rough spots inside of those big pipes, they offer resistance to the flow. This shows up as a loss of pressure that’s dependent on how long the pipes are and how many gallons per minute we expect them to carry. In this case we have really big pipes carrying the water to the little turbine, so the small amount of flow (current) required only causes a 1 PSI drop in pressure in each pipe. So when we put our pressure meter across the ends of the pipes at the caps, you’ll see that instead of a full 120 PSI of pressure, we only have 118 PSI. That’s an acceptable loss in this case since our little turbine is rated for pressures from 110 to 125 PSI and all is well.

Small Pipes Equal Big Pressure Loss

Imagine, however, what would happen if your plumber went cheap and installed really small feeder pipes to your turbine. Your pump would still be creating 120 PSI of pressure, but the current flow would be restricted a lot by the too-small feeder pipes. Consequently, rather than losing just 1 PSI of pressure, you would now lose 10 PSI of pressure with the same current flow as before. Since the top of the turbine has 10 PSI of pressure holding it back, and the bottom of the turbine only has 110 PSI to begin with, there’s only 100 PSI difference in pressure to drive the little turbine. Our turbine needs at least 110 PSI to operate properly, so now it’s starved for pressure and won’t spin fast enough to do its job. This same effect of pressure loss would occur with larger pipes over very long distances. Therefore, if we made our pipes in the first illustration ten times longer, that 1 PSI of pressure loss would then become 10 PSI of pressure loss.

Big Wires

Now let’s substitute a battery or generator for the pump, and an electric heating element in our coffee maker for the turbine. Our generator is hooked up to the outlet powering the coffee maker’s resistive heating element with really big wires. And just as in the water example, there will be a certain amount of resistance to the current flow. This resistance to current (flow) is what causes voltage drops to occur. How much voltage drop is dependent on the type of metal inside the wire (typically copper, sometimes aluminum), the diameter of the wire (remember that 10-gauge wire is thicker than 14-gauge wire) and how long the run of wire happens to be (50 feet of wire will lose twice as much voltage as 25 feet of wire).

In our case we’ve run a sufficiently heavy wire from the generator to the outlet, so there’s maybe only 1 volt of electrical voltage (pressure) lost on the way through the black wire. But since it has to return through the white wire, there’s another 1 volt of loss on the return trip. That means the bottom side of the heater in our diagram is getting 119 volts of electrical pressure while the top side is getting 1 volt of electrical pressure. Since meters and heaters only care about the difference in voltage applied across their inputs, we’re providing 119 minus 1 which equals 118 volts. Since our little heater is rated for operation with voltages as low as 110 volts, we’re still in good shape and your coffee will be done in time.

Small Wires

But now we’ve cheaped out and installed far too skinny of an extension cord from the generator to the heater outlet. And any time we try to pull a significant current flow (let’s say, 10 amperes) down the skinny wire, there’s a lot of resistance to that flow, causing us to lose electrical pressure (voltage) just as we lost water pressure when using the pump with too-small of connecting pipes. In our generator illustration above there’s a 10-volt drop in the black wire and a matching 10-volt drop in the white wire. That leaves our heater with 110 volts on the bottom feed and 10 volts on the top feed. Again, our meter and heater element only care about the voltage differential applied to them, so it’s only working with 100 volts.

Bad Things

That should sound like a bad thing to you, and indeed it is. Two significant problems occur when you hook up your RV using a long or skinny (or both) extension cord. The first is that this “electrical friction” causing the voltage drop makes the wire itself heat up. And it can heat up to the point where it gets limp and catches on fire. The second problem is that your RV is only getting 100 volts of electrical voltage (pressure), when it really wants 120 volts. You can reverse think this and realize that voltage drops only occur when you’re drawing significant amperage like an air conditioner or microwave. So while your electronic appliances such as a television may be operating properly with nothing else running in your RV, as soon as you turn on that roof air conditioner, you might see your television’s electronics starve for voltage and shut down. That’s certainly a problem if you’re watching an NCIS marathon.

Truth or Fiction: Low Voltage Kills Appliances

Well, low voltage affects only certain kinds of appliances and only under certain conditions. Resistive heaters like a coffee pot really don’t care if you feed them 120 volts or 110 volts or even 100 volts. They’ll just happily draw less current, which makes less wattage, which then takes longer to bring your water to a boil. And certainly roof air conditioner compressors can refuse to start if you don’t provide them with sufficient voltage and current. That motor has to push a piston against a lot of Freon gas pressure, and if you don’t have enough push it’s going to stall. However, there is one type of electrical load that can be severely damaged by running off of too low a voltage, and that’s an AC-DC motor with brushes like in a circular saw.

Advanced Concept Alert

Here’s why…. AC-DC motors put out a reversed voltage (called Back EMF for Electro Magnetic Field) when running at their designed voltages. That’s because as these motors spin they also act as generators feeding a reversed voltage back into their own electrical circuit. And that reverse voltage (pressure) is what holds back the current flowing through their brushes (the sparky things you see at the back of your drill). However, if you starve an AC-DC motor for voltage by using too small or too long of an extension cord, they won’t develop enough of this internal reverse voltage to limit the current flow through their own brushes. And that’s why you see lots of sparks fly from your power tools when running them on too long and too skinny extension cords. Reducing the voltage on an AC-DC power tool motor actually increases its amperage draw. You’ll kill the brushes in short order and ruin the motor in your hand drill or circular saw unless you maintain full voltage to the tools no matter what your amperage load happens to be. Thick extension cords make for happy power tools.

Back to Basics

Also, of course, letting your voltage drop below 110 volts is bad for computers, sound systems and virtually everything else in your RV. And while it may not cause an actual meltdown in your stereo, it will reduce the performance of virtually everything not fed with sufficient voltage. Just like trying to start your car’s engine when the battery is nearly depleted will leave you with a grrrr, grrr, grrr and no start, making your appliances run from too low of a voltage, which will sometimes make them shut down or not boot up properly.

Replay

Go ahead and re-read NSZ part V on current flow and make sure you have a heavy enough extension cord for the job without getting a big voltage drop. If you’re in doubt, go one size heavier (lower gauge number) for the wire size, especially if you’re running more than 25 feet of total length. There’s really no such thing as too thick of an extension cord.

Future Shock

Part VII of this series will cover how to calculate current draw from the various appliances in your RV. Stick around.

Feedback

After you’ve read this article at www.RVtravel.com, take a trip over to www.NoShockZone.org and send us your comments and suggestions. We’d love to know how we’re doing with this important project.

Mike Sokol is the chief instructor for the HOW-TO Sound Workshops (www.howtosound.com) and the HOW-TO Church Sound Workshops. He is also an electrical and professional sound expert with 40 years in theindustry. Visit www.NoShockZone.org formore electrical safety tips for both RVers and musicians. Contact him at mike@noshockzone.org.

6 Comments to “RV Electrical Safety: Part VI – Voltage Drop”

Wow, you are a great teacher. Thanks for sharing your knowledge. Voltage in my motor home runs at 122 then suddenly drops to 100-95. It will do this intermittently from every few hours to every few days. This happens with no AC on, just table fans and refer and lights. TV and cable receiver is not affected, but lights dim and table fans slow. I have a 30 amp service plugged into a 50 amp pedestal with a new adapter and a surge protecter (Progressive). The low voltage occurs even without the Progressive. I have new 12 volt batteries. Could a faulty converter or any component of the 12 volt system possibly cause this voltage fluctuation? Could a faulty 120VAC circuit breaker cause any fluctuation? I’m obviously desperate. Thanks for your help.

If you’re seeing that level of voltage drop WITHOUT drawing a lot of excessive amperage, then you’ve got to have a high-resistance connection somewhere in the 120-volt power line. So it could be a circuit breaker, or even the transfer switch. The key to finding problems like this to monitor the voltage in your RV while someone flexes, twists and vibrated every possible connection. But one thing to try first is to make sure you’re disconnected from AC power, then open up your transfer box and circuit breaker panel in your RV. Then retighten every electrical fitting you can find in the box. Be careful not to over-tighten. I usually loosen the screw a turn first, then tighten the screw until I can feel it seat the connection.
At least that’s a good place to start.

AC/DC or universal motors are NOT affected in the way described by low voltage. The only type of motor affected by low voltage conditions are Induction motors, where a drop in supply voltage ( below the design rating ) causes increased current, shortening the life of the device. You need to do some more research.

Can you provide references to your statement that AC/DC motors don’t exhibit Back EMF. I remember studying this in college back in the 70s, and just looked up Back EMF on Wikipedia today, and there’s a well referenced article which confirms the effect for AC/DC motors. http://en.wikipedia.org/wiki/Counter-electromotive_force Or maybe I’m just saying it incorrectly? Please elaborate.

I’ve been having intermittent issues with my fridge and furnace. The fridge has dual power options of either LPG or 120v. Surprisingly, the fridge requires 12v even when connected to shore power (120v). I’ve isolated this to the voltage entering the 12 VDC connectors on the back of the devices. The voltage appears to go down to 4v or 8v, but these appliances won’t operate without 12v. The batteries are fully charged and the voltage at the batteries shows acceptable levels. If I directly wire the battery bank to the fridge it runs fine using both the LPG and 120v.

This has to be a high resistance connection of some sort between the battery and the connection on the back of the appliance. First of all, how are you measuring that 12-volt line. Be sure that you’re metering directly on the appliance connector, then try putting the negative probe on the RV frame and measure again. A loose or corroded ground connection to the appliance can cause this sort of reading. If the drop is indeed in the 12-volt line, then it’s going to take some detective work. You’ll need to do a visual examination of the entire wiring run, looking for any terminal blocks or connectors in between. It’s also possible for a wire pinched between the frame or cabinets to break the copper wire internally causing this same reading. Let me know what you find out.

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